For some of us, there is a certain allure to owning an oddball wildcat. (I admit to being one of those loonies...) I have certainly built my share of the the oddballs. In fact, when I started with my 6.5x55's they
WERE an oddball.
I believe it was Bob Jourdan who did an article in
Precision Shooting magazine on the ackley improved cartridges. In the article he mentioned the 6.5x55BJ, his own version of the 6.5x55 AI. IIRC, he also mentioned that there is no known record of P.O. Ackley working with said case...
Operating, strickly from memory here: I seem to recall that he mentioned somewhere about a 4% increase in velocity. Which isn't all that much. Since each rifle is a law unto itself, you may or may not do as well.
For myself, there is not much reason to do it when one owns two 6.5 Rem Mags, two 6.5/06's, and a .260 Rem A.I.
But one thing I'd bet on in the long run, is that IF you do it, you'll likely look back on your efforts and realize that anything you are likely to hunt with it, you can kill just as dead with a 6.5x55.
And if the aforementioned isn't enough to scare you off... The price of a set of custom dies likely will. Last I knew, series "G" custom dies from RCBS are about $135, dealer cost. Redding: about the same, perhaps slightly less.
Another thing, you'll find that resale of said rifle will be only a fraction of what it costs to build it. And the dies; you'll be lucky to get 30% return on your investment.
On the positive side... Only you can put a price on your fun... If you think it's worth it, then go for it...
I, myself, have intended to build one for some time. I have yet to do it, tho'... And I'm sitting on a new barrel blank,
AND the dies... But ahead of that project, is a 6.5x54 MS... that is near completion, and a .256 Newton, that I've not even started yet....
Good luck in your endeavor